Automated formatting

Would there be any interest in introducing a `.clang-format` file for this library? It would not set precedent for boost - `gil`, `graph`, `histogram`, `multiprecision`, `nowide`, `outcome`, and `yap` all currently have .clang-format files. As far as I can tell, those are each unique format files, so I think there would be large latitude to use any configuration that makes sense for this library.